The present invention relates to an apparatus, a computer system and a computer program for determining a cardio-vascular parameter of a patient by thermodilution measurements.
The current state of the art in implementing transpulmonary thermodilution measurement are apparatus for injecting a bolus of thermal indicator into a patient""s vena cava superior, and measuring the temperature response at a place of the patient""s systemic circulation, e.g. patient""s arteria femoralis to determine the Thermodilution Curve, i.e. the temperature response as a function of time. From the thermodilution curve, a schematic example of which is illustrated in FIG. 1, wherein the abscissa (time axis) 1 is linear and the ordinate (temperature difference axis) 2 is logarithmic, various cardio-vascular parameters can be derived by using computersystems running computer programs, which implement parameter calculations as disclosed in WO 93/21823, the contents of which are included herein by citation, and as set forth briefly below.
The Cardiac Output CO can be determined by algorithms based on the Stewart-Hamilton-equation:   CO  =                              V          L                ⁡                  (                                    T              B                        -                          T              L                                )                    ⁢              K        1            ⁢              K        2                    ∫              Δ        ⁢                  xe2x80x83                ⁢                              T            B                    ⁡                      (            t            )                          ⁢                  ⅆ          t                    
where TB is the initial blood temperature, TL is the temperature of the liquid bolus, which is used as thermal indicator, VL is the thermal indicator volume, K1 and K2 are constants to consider the specific measurement setup, and xcex94TB(t) is the blood temperature as a function of time with respect to the baseline blood temperature TB. Thermal indicator can either be colder or warmer with respect to blood temperature. To obtain cardiac output, the area under the thermodilution curve has to be integrated.
Other parameters that can be derived from the thermodilution curve 3 as schematically illustrated in FIG. 1 include the Exponential Decay or Downslope Time DST, i.e. the time the blood temperature difference xcex94TB(t) takes to drop by the factor exe2x88x921, the Appearence Time AT, i.e. the time span between bolus injection IT and first appearence of a noticable temperature difference xcex94TB(t) and the Mean Transit Time MTT.
The Intrathoracic Thermovolume ITTV and the Intrathoracic blood volume ITBV can be determined as follows:
ITTV=COxc2x7MTT
ITBV=axe2x80x2xc2x7GEDV+bxe2x80x2
wherein axe2x80x2 and bxe2x80x2 are species-specific constants and GEDV is the Global End-Diastolic Volume, which can be determined as follows:
GEDV=COxc2x7(MTTxe2x88x92DST)
An extravascular thermovolume estimate can be determined as the difference between Intrathoracic Thermovolume ITTV and the Intrathoric blood volume ITBV
ETV=ITTVxe2x88x92ITBV
Extravascular thermovolume correlates, if there is no significant perfusion deffect in the lungs (e.g. pulmonary embolism), closely to the degree of Extravascular Lung Water. However, the clinical value of that measurement has not been shown explicitly yet.
A diagram similar to FIG. 1 is shown in FIG. 2 illustrating the problem of a baseline drift of the blood temperature. Again, the abscissa (time axis) 11 is linear and the ordinate (temperature difference axis) 12 is logarithmic. The baseline drift is indicated by baseline 14, the drift being shown excessive for the purpose of illustration. The schematically shown transpulmonary Thermodilution Curves 13, 15 with the same, constant Cardiac Output result from different boundary conditions. The first Thermodilution Curve 13 has been determined without the presence of a substantial extravascular thermovolume, whereas the second Thermodilution Curve 15 is broader and exhibits a less pronounced blood temperature peak due to the presence of a substantial extravascular thermovolume. The hatched area 16 illustrates the error of the area under the blood temperature curves 13, 15 and thus the error of the Cardiac Output determined from each curve due to the baseline drift. It is obvious, that determining Cardiac Output from the second Thermodilution Curve 15 will suffer from a significantly larger error due to baseline drift than determining Cardiac Output from the first Thermodilution Curve 13.
The object of the present invention is therefore to reduce the error in Cardiac Output determination due to a baseline drift, when a substantial extravascular thermovolume is present, and thus improve accuracy and reliability of determining cardio-vascular parameters by thermodilution measurements.
In order to accomplish the above mentioned object, the present invention provides an apparatus for determining a cardio-vascular parameter of a patient by thermodilution measurements comprising temperature influencing means for provoking an initial local temperature change in the proximity of a first place of a patient""s vascular system thus introducing a travelling temperature deviation to patient""s blood stream, further comprising a temperature sensor device for measuring the local temperature of patient""s blood at a second place of patient""s vascular system downstream of the first place, further comprising a computer connected to the temperature sensor device for recording the patient""s local blood temperature measured at the second place as a function of time to determine a thermodilution curve, determining an extravascular thermovolume estimate from the thermodilution curve, determining a new initial local temperature change depending on the thermovolume estimate, controlling the temperature influencing means to provoke the new initial local temperature change in the proximity of the first place, determining an improved thermodilution curve, and determining the cardio-vascular parameters from the improved thermodilution curve.
In order to accomplish the above mentioned object, the present invention also provides a computer system comprising first connection means to connect the computer system to temperature influencing means and second connection means to connect the computer system to a temperature sensor device, and accessing means to acces executable instructions to cause the computer system to control temperature influencing means connected to the computer system to provoke an initial local temperature change in the proximity of a first place of a patient""s vascular system, thus introducing a travelling temperature deviation to patient""s blood stream, to record the patient""s local blood temperature measured by a temperature sensor device at a second place of patient""s vascular system downstream of the first place as a function of time to determine a thermodilution curve, to determine an extravascular thermovolume estimate from the thermodilution curve, to determine a new initial local temperature change depending on the thermovolume estimate, to control the temperature influencing means to provoke the new initial local temperature change in the proximity of the first place, to determine an improved thermodilution curve, and to determine the cardio-vascular parameters from the improved thermodilution curve.
In order to accomplish the above mentioned object, the present invention also provides a computer program for determining the cardio-vascular parameters of a patient by thermodilution measurements comprising instructions executable by a computer system to cause the computer system to control temperature influencing means connected to the computer system to provoke an initial local temperature change in the proximity of a first place of a patient""s vascular system, thus introducing a travelling temperature deviation to patient""s blood stream, to record the patient""s local blood temperature measured by a temperature sensor device at a second place of patient""s vascular system downstream of the first place as a function of time to determine a thermodilution curve, to determine an extravascular thermovolume estimate from the thermodilution curve, to determine a new initial local temperature change depending on the thermovolume estimate, to control the temperature influencing means to provoke the new initial local temperature change in the proximity of the first place, to determine an improved thermodilution curve, and to determine the cardio-vascular parameters from the improved thermodilution curve.
In order to accomplish the above mentioned object, the present invention also provides a storage medium having physically stored thereon a computer program for determining the cardio-vascular parameters of a patient by thermodilution measurements comprising instructions executable by a computer system to cause the computer system to control temperature influencing means connected to the computer system to provoke an initial local temperature change in the proximity of a first place of a patient""s vascular system, thus introducing a travelling temperature deviation to patient""s blood stream, to record the patient""s local blood temperature measured by a temperature sensor device at a second place of patient""s vascular system downstream of the first place as a function of time to determine a thermodilution curve, to determine an extravascular thermovolume estimate from the thermodilution curve, to determine a new initial local temperature change depending on the thermovolume estimate, to control the temperature influencing means to provoke the new initial local temperature change in the proximity of the first place, to determine an improved thermodilution curve, and to determine the cardio-vascular parameters from the improved thermodilution curve.
In a preferred embodiment of the present invention, the temperature influencing means is an injection means for injecting a liquid having a temperature different from the temperature of patient""s blood, provoking the initial local temperature change is achieved by the injection means injecting at the first place a first amount of liquid into the vascular system, the liquid having a first temperature different from the temperature of patient""s blood, and provoking the new temperature difference is achieved by the injection means injecting at the first place a second amount of liquid into the vascular system, the liquid having a second temperature different from the temperature of patient""s blood.
In another preferred embodiment of the present invention, the second temperature is different from the first temperature.
In another preferred embodiment of the present invention, the second amount is different from the first amount.
In another preferred embodiment of the present invention, the cardio-vascular parameter is determined by transpulmonary thermodilution and the extravascular thermovolume estimate correlates to an estimate of Extravascular Lung Water. Extravascular thermovolume correlates, if there is no significant perfusion deffect in the lungs (e.g. pulmonary embolism), closely to the degree of Extravascular Lung Water. However, the clinical value of that measurement has not been shown explicitly yet.
In another preferred embodiment of the present invention, the extravascular thermovolume estimate is determined from a cardiac output estimate derived from the thermodilution curve, a downslope of the thermodilution curve, and a mean transit time estimate derived from the thermodilution curve indicating an estimate of the time required by the temperature deviation to travel from the first place to the second place.
In another preferred embodiment of the present invention, the cardio-vascular parameter to be determined is the cardiac output.
The accompanying drawings serve for a better understanding of the above and other features of the present invention.